The instant invention relates generally to a furnace for heating air or water for heating, for example, a building, the furnace primarily intended for the burning of wood. More specifically, the instant invention incorporates an improved secondary combustion system into such a furnace.
A simple wood burning stove or furnace usually comprises a metal box having a door for loading wood, an air inlet control system (often part of the door) for controlling the amount of combustion air admitted into the box and an exhaust flue for directing exhaust gases from the box. Such simple wood burning stoves tend to be inefficient because unburned vapors and particulates pass out the exhaust flue. Admitting more combustion air may reduce the amount of unburned vapors and particulates passed out the exhaust flue but then the fire tends to burn too hot and too fast.
A solution to such problems is to promote xe2x80x9csecondary combustionxe2x80x9d. Primary combustion is throttled by controlling the amount of combustion air. The unburned vapors and particulates are then mixed with heated air to burn such vapors and particulates and thereby recover more heat and reduce pollution. U.S. Pat. No. 4,672,946 (herein fully incorporated by reference), for example, described the use of a perforated tube to admit secondary combustion air with the unburned vapors and particulates and then promoted secondary combustion with an insulated combustion device. As a further example, U.S. Pat. No. 5,462,043 (herein fully incorporated by reference) also used a perforated tube system to admit secondary combustion air. However, the prior art secondary combustion systems tend to be too expensive to manufacture or require too much maintenance or require a relatively narrow range of operating conditions.
The instant invention provides a relatively inexpensive, rugged and maintenance free system for secondary combustion in a wood burning furnace that is effective over a broad range of operating conditions. More specifically, the instant invention is a wood burning furnace, comprising: (a) an enclosed combustion chamber including a top, a bottom, a back, a front and opposing side walls; and (b) a secondary combustion air admission chamber including a top, a bottom, a back, a front and opposing side walls, the bottom wall of the secondary combustion air admission chamber being arched and perforated, the bottom wall of the secondary combustion air admission chamber comprising the top wall of the combustion chamber. A combustion air blower is preferably in fluid communication with the secondary combustion air admission chamber so that combustion air entering the secondary combustion air admission chamber passes through the perforations in the arched bottom wall of the secondary combustion air admission chamber and into the combustion chamber. In addition, an air distribution manifold is preferably disposed within the combustion chamber, the air distribution manifold having openings disposed to direct air passing therethrough toward the bottom of the combustion chamber, the air distribution manifold being in fluid communication with the combustion air blower.